While that's still a relatively low proportion of the population, it represents a 58% increase over a two-year period.

But one of the key factors is the age of those listening. Recent research from podcast platform Acast suggests that podcast listeners in the UK tend to be millennials - with two-thirds falling into the 16-34 age bracket. Of this group, 27% listen once a week, and 29% listen between two to four times a week.

What's more, 21% of respondents in the Acast research, conducted by Ipsos Mori, said they had started listening in the last six months.

The popularity of podcasts with this elusive audience - which so many traditional broadcasters and media companies have failed to attract - may be one of the key drivers of the growing podcast market.

"This is the year podcasts have gone mainstream," says Hannah Verdier, a freelance podcast reviewer. "A couple of years ago it was still a really niche thing. It seemed to me that no-one really listened to podcasts outside of the London media bubble.

"But this year feels different, and the last few weeks, in particular, they've really hit their stride.

"The one that has really exploded has been the Love Island podcast - it's been on the top of the charts since it launched. It's now seen as essential to a brand to have a podcast as well as a website."

'A podcast boom'

Matt Deegan, who produces the podcast Love Island: The Morning After, says: "In the last six months a lot of new podcasts have emerged, so it would be fair to say it's having a bit of a boom.

"We're seeing lots of tweets and reaction to our podcast - many of which are from college students who say they are listening to a podcast for the first time.

"The successful podcast format has traditionally been the companion format - fan-made shows, for the likes of Westworld, West Wing etc.

"Spin-off TV shows have always been popular - like Big Brother's Little Brother, and the same goes for podcasts. Love Island is the first UK television show to really go for it and I think they have been rewarded for it."

Podcasts, as we know them, have been around for 15 years and gained some traction in the mid-2000s but seemed to take a blow from other online media content.

Rowan Slaney, who writes the Guardian's Hear, Here podcast column, and is an audio content specialist at Google, says: "YouTube came along and since then everyone's been pretty video-focused. And then slowly - mainly because American speech radio is awful - podcasts have been making a comeback.

"The UK has typically been behind the US in the podcasting stakes because the BBC makes good content and people don't feel the need to go away and download different audio content.

"But now everyone has a podcast - and the BBC has had to start taking it more seriously too."

'True crime craze'

One of the landmarks on the podcast landscape was, of course, the podcast Serial. The first two seasons of the true crime hit have been downloaded more than 250 million times, according to Serial Productions.

Image copyrightSerialImage caption
Serial is about the conviction of Adnan Syed for the murder of ex-girlfriend Hae Min Lee

"Serial was amazing," says Slaney. "But you can't attribute everything to that podcast. If it weren't for the ongoing dedication of real audio fans, who had the interest in podcasts all along we would never have had a series like that. Serial kicked off a real true crime craze - and it's still the most-listened-to genre."

Deegan, who is also the co-founder of the British Podcast Awards, says Serial coincided with podcasts becoming much more accessible via smart phones.

"When it became a default app on the iPhone, perhaps four or five years ago, it meant more and more people clicking on it. And that was the same sort of time Serial came out - which was the first real podcast content hit."

'Game changer'

It wasn't much longer before the podcast sensation My Dad Wrote A Porno came out. Hosted by Radio 1 presenter Alice Levine and two of her university friends, James Cooper and Jamie Morton, the podcast has been on the iTunes chart for more than two years. It recently went on tour with its live shows, selling out the Royal Albert Hall.

Image copyrightPAImage caption
The stars of the podcast My Dad Wrote a Porno, l-r: James Cooper, Jamie Morton and Alice Levine

"When My Dad Wrote A Porno came along, it was a game-changer," says Hannah Verdier. "And others have realised there is a real market there with young, female audiences."

Slaney agrees: "Everything is traditionally made for men. Young women are let down by most speech radio - there's a real gap.

"Currently the only real route into audio is through the BBC and if you don't fit into those six channels then there's nowhere for you to be heard on mainstream media. But podcasting is a really exciting format, and anyone can do it - so it opens up the door to lots more content and more creativity.

"Now young women are making their own content that they enjoy. And the amount of black and non-white voices we've heard in podcasting has just exploded.

"They're also voices that are not well served by traditional media, so it's a means for ethnic minorities to get their voices heard."

The Shout Out Network is one of a number of platforms supporting people of colour and actively encouraging women in podcasts. Wolves In The City, hosted by YouTuber Lee Gray, grime artist Karnage Kills and DJ Jay Jay Revlon, is just one of the podcasts they promote - which aims itself at gay black men exploring their sexuality in London.

Image copyrightTwitter/@WolvesITCImage caption
The team behind Wolves in the City

And celebrities have seen fit to get in on some of this action too.

Deegan says: "Probably in the last six to 12 months, we've seen a higher number of mainstream personalities getting into the world of podcasts, like Fearne Cotton, Ed Miliband, Jessie Ware, along with a number of others."

The success of some of these more recent podcasts has in turn informed the kind of radio that is now being commissioned.

"I think podcasts have opened radio's mind," says Deegan. "Radio 4 has commissioned a series of the Beef and Dairy Network, for example, a comedy which started out as a podcast."

And Ed Miliband has sat in twice for Jeremy Vine on Radio 2 following the success of his Reasons to be Cheerful podcast - currently number 16 on the iTunes chart.

"In short," says Deegan, "podcasts have grown up."

Podcasts to listen to now...

Hannah Verdier's recommendations:

Image copyrightGetty ImagesImage caption
Julia Davis and Vicki Pepperdine's Dear Joan and Jericha is currently number two in the podcast charts

Dear Joan and Jericha - Julia Davis and Vicki Pepperdine pretend to be a pair of agony aunts and they're quite rude and filthy. It's got a really dry, observational humour. Very daring.

Gossip - about three friends who meet weekly to... you guessed it... gossip and freak out over the latest developments in their fictional suburban town, Golden Acres.

Dirty John - this is a good one for total podcast beginners. It's an American true crime about a woman who meets a man on the internet. She thinks he's a surgeon and then it all unravels...

Sandra - this is a bit more upbeat from Gimlet, one of the best podcasting companies in the US. Ethan Hawke plays a boss and Kristen Wiig plays an Alexa/Siri type character. Turns out Sandra is powered by real people though. Dark comedy drama.

Griefcast - this doesn't sound fun, but trust me, it's good. It's very simple: just people talking about their experience with grief. It's really honest but uplifting and a really nice listen.

Rowan Slaney's recommendations:

Capital - a drama about a referendum won by 51% of the vote to bring back capital punishment.

The Discovery Adventures - a good one for the family. It's easy and fun, and you can go and visit some of the places that the episodes are based on.

Harry Potter and the Sacred Text - two friends going through each chapter of Harry Potter, looking at the symbolism and language of all the books. Very well structured.

The Dollop - two comedians find the most ridiculous events in US history that shaped their country. Really well researched and funny.

How To Be A Girl - won the best international podcast at the British Podcast Awards. A mother and her transgender daughter talk to each other. It's about how they navigate the world while she's growing up and going through her transition.

Matt Deegan's recommendations:

Image copyrightTwitter/@walkthedogcastImage caption
The Times' Emily Dean interviewed Alan Carr in her first episode of Walking the Dog

Where should we begin? - Esther Perell is a counsellor. You sit in a therapy situation with real couples.

The Tip-Off - behind the scenes of investigative journalism, how did they get the story?

Love Island - you should definitely listen to this if you like the show,

Pod Save America - Barack Obama's old speech writing team, on the annoying things Donald Trump has done that week. Has turned from a very angry podcast, to more of a campaigning, fun podcast.

Walking the Dog - from The Times. Emily Dean interviews a celebrity guest each week while they walk their dogs.